For improved miniaturization or higher performance of a transistor, a vertical transistor having a gate electrode buried in a trench is often used. In the vertical transistor, drain-source breakdown voltage (hereinafter, also referred to as “breakdown voltage”) and ON resistance are in a trade-off relationship. That is, if impurity concentration of a drift layer increases to decrease the ON resistance, the breakdown voltage decreases. Conversely, if the impurity concentration of the drift layer is decreased to increase the breakdown voltage, the ON resistance increases.
There is a structure in which a field plate electrode is provided below a gate electrode in a trench, which is used as a method of improving the trade-off relationship between breakdown voltage and ON resistance. An electric field distribution of a drift layer is changed by the presence of a field plate electrode, whereby a breakdown voltage of a vertical transistor can be increased while an impurity concentration of a drift layer is maintained. However, further improvement of the trade-off relationship between the breakdown voltage and the ON resistance of the vertical transistor is required.